[Builder] Eliminate the StringRef/StringAttr forms of getSymbolRefAttr.

The StringAttr version doesn't need a context, so we can just use the
existing `SymbolRefAttr::get` form.  The StringRef version isn't preferred
so we want to encourage people to use StringAttr.

There is an additional form of getSymbolRefAttr that takes a (SymbolTrait
implementing) operation.  This should also be moved, but I'll do that as
a separate patch.

Differential Revision: https://reviews.llvm.org/D108922